In many applications for dispensing of flowable materials and liquids, it is required to be able to start and stop the dispensing action many times. Many dispensers or applicators of flowable materials provide some form of shut-off valve, but these are often of only limited application. In many cases, the valves in such dispensers are not efficient even working under optimum conditions. Even after the valves are shut off, residues of material will cling to the nozzle, or drip onto the workpiece or machinery. With thicker, more viscous materials or liquids, the problem becomes more aggravated. With many designs of shut-off valves, even after the valve has been closed, the material or liquid strings out from the nozzle, or clings to the nozzle, resulting in the placement of an inaccurate deposit of the material on the workpiece, or leaving material stringing from the dispensing head to the machinery or to the workpiece, in places where it is not required.
While a wide variety of viscous materials and liquids may require dispensing in this way, the dispensing of adhesives and bonding agents presents these problems in a particularly acute form. More specifically, in some high-volume, high-speed gluing systems, some build-up of glue on and about the nozzle or outlet of the dispenser or applicator member has been experienced. While this may be corrected by physical wiping or cleaning by an operator, it is quite difficult to correct this condition while the gluing system is in operation. At the relatively high speeds at which most systems are operated, it is quite difficult for an operator to physically wipe or otherwise clean the dispensing applicator or nozzle without interfering with the relatively high speed dispensing of glue thereby. However, it is also undesirable to shut down a relatively high speed, and highly efficient gluing system and attendant article handling lines and equipment at frequent intervals to achieve such cleaning or wiping operations.
Valve or nozzle assemblies of the general class herein, having a suck-back valve or draw-back capability after closing the discharge, include some type of discharge valving, and a chamber downstream thereof in communication with the nozzle for effecting a suck-back flow of liquid from the nozzle upon an increase in volume in the chamber. The simplest approach includes the use of a hand-operated plunger which forces the liquid, such as an adhesive, out of a discharge nozzle from an adjoining chamber containing the liquid, and subsequent retraction of the plunger draws-back product from the nozzle to prevent product extrusion from the nozzle tip after a predetermined amount is dispensed or applied to a workpiece. This has proven largely ineffective for accurately cutting off the flow of product through the nozzle for liquids of different viscosities, and is obviously unsuitable for large scale dispensing operations. Other valve arrangements provide for a more precise cutting off of the flow of product from the nozzle after a closing of the discharge valve. However, many of such arrangements require relatively movable parts to effect a suck-back flow from the nozzle, or separate elements for closing the discharge and thereafter creating the suck-back flow. Otherwise, external means are required for carrying out the discharge closing and/or draw-back operations. These various prior art approaches thus require numerous parts and external functions which add to the complexity and cost of the valve arrangement and are more cumbersome to operate.